Add draft_id variable to event core.posting_modify_template_vars

The draft_id is being sent through in the events 'core.modify_posting_parameters' and 'core.modify_posting_auth', however these are before the value of draft_id is actually checked (lines 784 and below in posting.php).

For extensions it could be beneficial to also be able to receive the checked $draft_id. In particular, this would be the case for this extension:
https://www.phpbb.com/community/viewtopic.php?t=2279486&view=unread#unread
. My proposal would be to add it to the event core.posting_modify_template_vars